1. Field of the Invention
The present invention generally relates to the formation of electrically conductive mechanical bonds at low temperatures and, more particularly, to the electrical and mechanical stabilization of conductive adhesives used in the formation of such bonds.
2. Description of the Prior Art
For many years, solder has been the principal material of choice for forming bonds between metal or other conductive materials. Many techniques and materials such as fluxes having very selective surface wetting properties have been developed to extend solder connections into integrated circuit and multi-chip module technologies where only very small separations are provided between conductors which must remain electrically isolated while solder joints are formed on the surfaces thereof.
Most materials used for the solder connections themselves are mixtures or alloys of tin and lead. These materials have melting points in the range of 220.degree. C. or higher. While this temperature range is relatively low and easily achieved in a great variety of ways, there are a number of materials and devices which can be damaged by such temperatures. With regard to some applications, heat sinking with local application of heat may provide a solution to the successful formation of a solder bond. However, the use of a heat sink generally implies the potential for application of a greater amount of heat than is tolerable since the performance of the thermal connection to the heat sink is somewhat unpredictable.
Therefore, for close control of temperature, general heating in a furnace or oven of the device with a material for forming the bond in place is the technique of choice. Of course, with general heating, there is no possibility of protection of any temperature sensitive material which may be present and damaged by the maximum temperature attained. If the temperature at which a material may be damaged is lower than the melting point of the solder material, as is the case with recently developed low dielectric constant materials, a solder bond is simply not appropriate and cannot be formed without loss of the properties for which the low dielectric constant material may have been chosen.
Conductive adhesives are known for forming electrically conductive mechanical connections. Such conductive adhesives generally involve a high concentration of conductive plastics in a thermosetting or thermoplastic binder. Conduction, in any case, depends on compressive force between the particles to maintain an intimate surface contact therebetween. Such a compressive force is generally obtained through volume shrinkage as thermosetting binder cures or through joining reflow with thermoplastic binders.
In either case, however, the binder matrix is subject to a tendency to creep during thermal cycling. As a result, the compressional force may be lost and the mechanical contact between some particles may be broken; causing a resistance drift over time or at least instability of resistance of a connection. In severe cases, complete loss of electrical integrity of some connections results, often unpredictably and after a particular device or circuit has been placed in service.
In summary, connections made with conductive adhesives have been generally unreliable due to the tendency for compressive force between the conductive particles to be slowly lost or severely compromised over time due to even moderate thermal cycling. Since contact between the particles is generally over a very small area because the particles are generally spherical, loss of reliable compressive force between even a relatively small fraction of the particles can cause substantial fluctuations in resistance of the connection, especially during thermal excursions. Therefore, instability of resistance can significantly change circuit performance even long before continuity is lost.